As a magnetic sensor for detecting a leakage magnetic field existing in and/or out of a train or an automobile, a search-coil type magnetic sensor operating by virtue of electromagnetic induction was conventionally used. However, a magnetic sensor including a coil could not detect a DC magnetic field.
A fluxgate type apparatus having been practically employed for detecting magnetism is able to detect not only a DC magnetic field, but also an AC magnetic field. However, an effective range of a frequency of a detectable AC magnetic field is merely a few kHz at maximum. A fluxgate type magnetism-detecting apparatus was not capable of detecting not only a DC magnetic field, but also an AC magnetic field having a frequency of about 100 kHz at maximum. This is because it is extremely difficult to convert a magnetic field in the range of a DC magnetic field to an AC magnetic field having a frequency of about 100 kHz and having the same magnetic field strength as that of a DC magnetic field, into electric signals having a constant strength, and further, it is also extremely difficult to guarantee the performance to do so.
In these years, there were fabricated a lot of trains from which a strong leakage magnetic field was generated, and it is afraid that a leakage magnetic field exerts harmful influence onto human beings and/or magnetic storage medium, and accordingly, “Railway rolling stock-Measuring methods of leakage magnetic field (JIS E 4018)” was defined by Japan Industry Standards Research Committee.
The method defines objects to be measured, and conditions under which measurement is to be carried out. The objects include a leakage magnetic field (magnetic flux density), and devices from which a magnetic field is generated, both existing in and out of a train. The conditions are defined in accordance with a status of a train. For instance, while a train is running, a leakage magnetic field in a train and in the vicinity of a device generating a magnetic field should be measured at a speed range of a train at which a maximum current runs through the device. Since a DC magnetic field having a magnetic flux density in the range of about 1 to about 2 mT is measured when a train starts running, a measurement device including Hall element is employed.
Specifically, X-, Y- and Z-components of a magnetic field are measured by means of a measurement device having an accuracy of about ±5%, the measured components are synthesized in accordance with the equation (1), and a magnetic flux density is expressed with the synthesized components. In recording the measurement results, a magnetic flux density is recorded in the form of both a synthesized density and components in each of axes.B=(Bx2+By2+Bz2)1/2  (1)
In the measurement of a magnetic field, X-, Y- and Z-components are basically concurrently measured. Since a conventional measurement device is a generally used device displaying an effective value or a wave-height value, performances for accomplishing instantaneous measurement of a waveform, and wide band frequency characteristics are not guaranteed, and accordingly, a synthesized value in measurement of an AC magnetic field was calculated in accordance with the equation (1), based on effective values associated with X-, Y-, and Z-axes or wave height values. As a result, since a maximum value of synthesized magnetic fields is calculated with both data simultaneity and phase relation among X-, Y-, and Z-components being ignored, the thus calculated value is not consistent with a true total magnetic force (a strength or an absolute value of a magnetic field vector) to be calculated with instantaneous values of X-, Y-, and Z-axes
This is because displayed effective values or wave-height values do not put data indicative of a phase relation among X-, Y-, and Z-components into consideration. For instance, a strength of a magnetic field (a strength of a magnetic field vector) calculated based on displayed wave-height values is always greater except particular cases than a true total magnetic force calculated based on data obtained when the X-, Y-, and Z-components are simultaneously measured, and has an error equal to or greater than a couple of tens %, which is remarkably beyond an allowed accuracy of ±5% of a measurement device. As a result, a synthesized value of a magnetic field calculated based on a displayed wave-height value is remarkable different from a true strength of a magnetic field vector due to an error caused by ignorance of a phase relation.
In another point of view, a conventional measurement device for measuring a magnetic field based on displayed effective values or wave-height values is of a device disregarding a distortion and/or a phase relation in waveforms of a magnetic field, and measuring an average with respect to a time as a strength of a magnetic field, and never guaranteed both instantaneous response performance to a magnetic field have high frequency components, and characteristics of accurately reproducing waveforms of a measured magnetic field.
A frequency band of a magnetic field generated from an automobile and a train broadly covers a magnetic field in the range of a DC magnetic field to an inverter frequency, and a high-frequency noise magnetic field caused by switching. In order to analyze these magnetic fields with FFT (Fast Fourier Transform), it is necessary to use a wide-band type device for measuring a magnetic field as a practical device capable of measuring, with a constant detection sensitivity, magnetic fields including not only a low-frequency band such as a DC magnetic field, a variable magnetic field, and a magnetic field for a commercial frequency, but also a high-frequency band of about 100 kHz.
Furthermore, the wide-band type device for measuring a magnetic field is required to have a remarkably wide dynamic range, specifically, to be able to measure magnetic fields ranging from a strong magnetic field of a couple of mT to a weak magnetic field in the range of hundreds of nT to tens of nT which is afraid to be exert a harmful influence to human bodies.
A system for detecting magnetism in a magnetic sensor includes, in dependence on a theory for measuring magnetism, a system suitable for measuring from a DC magnetic field to a DC variable magnetic field of a couple of Hz, a system suitable for measuring from a DC magnetic field to a DC variable magnetic field of hundreds of Hz, a system capable of measuring only an AC magnetic field in the range of a couple of Hz to tens of kHz, a system capable of measuring only a weak magnetic field, a system capable of measuring only a strong magnetic field, and so on.
For instance, a Hall element type magnetic sensor has a practically effective accuracy of about tens of μT, and accordingly is suitable for measuring a strong magnetic field, since a small magnetic field of about tens of μT may be ignored as an error when a strong magnetic field in the range of about 1 to about 2 mT is measured. However, when a weak leakage magnetic field of a couple of μT or less which is afraid to exert a harmful influence onto human bodies is measured, an error is greater than signals, and hence, signals indicative of a weak magnetic field are mixed with noises, and thus, cannot be found. Thus, the Hall element type magnetic sensor has merits and demerits.
Thus, a technique was invented in which a low frequency band and a high frequency band both including a DC magnetic field are measured by means of two types of magnetic sensors, respectively.
Specifically, the patent document 1 by the title of “An apparatus for and a method of measuring a magnetic field in railway rolling stock” discloses a complex type magnetic sensor including a combination of a magnetic oscillation sensor and a search coil type magnetic sensor, both of which complement shortcomings of each other to thereby be able to measure a wide band magnetic field. Herein, the magnetic oscillation sensor belongs to a fluxgate (IEC 61786 Definition of Standard) measuring a magnetic field by virtue of non-linear magnetic characteristics of a probe or a sensing part having a ferromagnetic core.
More specifically, the complex type magnetic sensor includes, as a first three-axis magnetic sensor, a search coil type sensor being good at detecting an AC magnetic field having a frequency of tens of Hz or greater, and, as a second three axis magnetic sensor, a magnetic oscillation sensor suitable for measuring a DC magnetic field or a variable magnetic field. By combining strong points of these two types of magnetism detection systems, the complex type magnetic sensor has no objects which cannot be measured by itself.
Each of the first and second three-axis magnetic sensors is designed to include a magnetism sensing part having a magnetism detection axis (a direction in which a magnetism sensing part senses maximum magnetism). Three magnetism detection axes are arranged to be perpendicular to one another to thereby make it possible to detect an external magnetic field by separating the external magnetic field into X-, Y-, and Z-components.
FIG. 7 illustrates a magnetic sensor disclosed in the patent document 1, having a basic construction in which each magnetism sensing part is housed in and is integral with a sensor casing.
A first three-axis magnetic sensor 51 includes a magnetism sensor for measuring only an AC magnetic field. The magnetism sensor is comprised of three search coils perpendicular to one another. Magnetic field signals (inductive voltages) detected by the search coils arranged in X-, Y-, and Z-axes are transmitted to a main measurement unit through a sensor cable 53, processed in a signal circuit, and then, output.
A second three-axis magnetic oscillation sensor 52 is comprised of a magnetic oscillation sensor for measuring a DC magnetic field and a low-frequency magnetic field. The magnetic oscillation sensor includes a magnetism sensor having three core-coils each including a magnetic core made of a ferromagnetic material. Magnetism detection axes of the core-coils are arranged along X-, Y-, and Z-axes such that they are perpendicular to one another.
FIG. 8 illustrates a basic circuit of a magnetic oscillation sensor as a device for measuring a magnetic field in three axes. In FIG. 8, 100 indicates an X-axis circuit part, 104 indicates a magnetism sensing part for an X-axis, 200 indicates a Y-axis circuit part, 204 indicates a magnetism sensing part for a Y-axis, 300 indicates a Z-axis circuit part, and 304 indicates a magnetism sensing part for a Z-axis. Since the circuits for three axes have the same structure as one another, the X-axis circuit part 100 is explained hereinbelow.
The magnetic oscillation sensor has a variation circuit of a multi-vibrator. Specifically, a variation circuit of a multi-vibrator is reconstructed to be able to oscillate, by replacing fluctuation in a voltage between capacitor terminals, that is, repetition of voltage fluctuation when oscillated, with a phenomenon of particular fluctuation in a voltage between terminals of a core-coil having non-linear characteristics when an AC current runs therethrough.
Since the oscillation in a multi-vibrator circuit is generated by virtue of non-linear excitation characteristics of a magnetic material, the oscillation circuit is called “a magnetic oscillation circuit”, and a magnetic sensor to which the magnetic oscillation phenomenon is applied is called “a magnetic oscillation sensor” or “a magnetic oscillation type magnetic sensor”.
An oscillation current running through the magnetic oscillation circuit passes through a core-coil 105, and accordingly, excites a core 106 alternately in a positive or negative direction to thereby magnetically saturate the core 106.
The oscillation current is therefore called also “an excitation current”.
The magnetic oscillation sensor in an X-axis circuit includes a magnetism sensor 104 comprised of a core-coil 105 including a core 106 as a magnetic core, an operational amplifier 108, and resistors 107, 109 and 110 electrically connected to the operational amplifier 108. The core-coil 105 includes a terminal P20 electrically connected to a non-inverted input terminal of the operational amplifier 108, and is grounded at the other end. The reference number 111 indicates a low-pass filter having a main function of attenuating magnetic oscillation frequency components included in a magnetism detection signal. The reference number 112 indicates an amplifying circuit which controls an amplitude of a voltage in accordance with a strength of an external magnetic field detected by the magnetism sensor, and outputs the thus controlled voltage through a terminal Q10.
If only an excitation magnetic field generated by an oscillation current is applied to the core 106, an excitation duration necessary for the core 106 to be magnetically saturated in a positive direction is equal to an excitation duration for the core 106 to be magnetically saturated in a negative direction, because of symmetry about an origin with respect to magnetization characteristics (B-H curve) of a magnetic material.
In another point of view, since an origin from which the core 106 starts its action is an origin of coordinate axes of B-H curve, positive and negative excitation durations necessary for the core 106 to be magnetically saturated in positive and negative directions are equal to each other, and thus, a time difference is equal to zero. Accordingly, an integration of an output voltage having a rectangular waveform in the operational amplifier 108 is equal to zero.
However, if an external magnetic field is applied to the core 106 under the above-mentioned condition, the external magnetic field overlaps an excitation magnetic field. As a result, an action point is shifted in a degree defined by a strength of the external magnetic field from an origin of coordinate axes of B-H curve, which is an origin from which the core starts its action, and hence, a gap is caused in timing at which the core is positively or negatively magnetically saturated. Specifically, a ratio between a positive half-cycle duration and a negative half-cycle duration (called “a duty ratio”) in the core is varied due to the external magnetic field, and thus, an integration of an output voltage from the operational amplifier 108 also varies accordingly
In other words, the external magnetic field is detected by a magnetic oscillation sensor as a fluctuation in an integration of an output voltage from the operational amplifier 108
An oscillation frequency of a magnetic oscillation sensor is initially adjusted by varying a partial voltage ratio between the resistors 109 and 110 both electrically connected to an output terminal of the operational amplifier 108 (adjustment at shipment).
However, such circuit structure as mentioned above is not ideal for the following reasons.
The first reason is that if a difference is caused in oscillation frequencies of a plurality of magnetic oscillation sensors, a signal is generated having a beat frequency (a “beat” frequency generated when two waves having frequencies slightly different from each other overlap).
In other words, a signal having a beat frequency component and not existing in an external magnetic field overlaps a detection signal as noises. It is difficult to identify a beat frequency component from a magnetism detection signal transmitted from a magnetic oscillation sensor, as a magnetic field having a beat frequency component cannot help from being recognized as an external magnetic field. Furthermore, if such phenomenon occurs, an output transmitted from a magnetic oscillation sensor will contain a fluctuation error even in a DC level in the range of about tens of nT to about thousands of nT in dependence on a strength of a disturbance magnetic field, resulting in that an environmental magnetic field cannot be accurately measured or a magnetic field cannot be measured in a strong field.
A magnetic oscillation sensor under conditions of being in a strong magnetic field has a tendency that a magnetic oscillation frequency lowers while a magnetic field is being measured, and hence, beat phenomenon readily occurs due to the fluctuation of the frequency, which is a serious defect which cancels various merits of a magnetic oscillation sensor with respect to its performances.
The second reason is that an accuracy with which a disturbance magnetic field is measured is degraded due to electromagnetic noises generated among core-coils in a three-axis magnetic oscillation sensor or electromagnetic noises generated in a neighboring search coil type magnetic sensor.
Thus, it was necessary to separate magnetism sensing parts and circuit parts in the three axes from one another, or space magnetism sensing parts in magnetic sensors from one another, when they are arranged in a sensor case. Specifically, magnetic oscillation sensors are randomly positioned with a sufficient space being among them, core-coils or search coils are randomly positioned with a sufficient space being among them, and/or a sensor case in which the sensors are housed is designed to be big enough to house magnetic sensors therein.
However, since a magnetic field is measured at each of positions of sensors in the above-mentioned solution, points at which a magnetic field is measured randomly exist, causing deterioration in accuracy of magnetic field measurements, and likelihood of measurement error is increased.
There is no problem in measurement of a magnetic field regardless of random positions of magnetic sensors, if a magnetic field is a uniform parallel magnetic field. However, in measurement of a magnetic field locally distorted with a steep disturbance of the strength of the magnetic field in or out of a train or an automobile, intensities of magnetism may be quite different from one another in dependence on random positions of magnetic sensors, resulting in measurement errors unavoidably caused due to positional gaps of magnetism sensing parts of magnetic sensors, and hence, measured intensities of magnetic fields are not reliable.